To add color to a photo, all you have to do is add markings to the different regions of the image, indicating how it should be colorized. Then click the Colorize button and let the software do the rest.

Recolored looks pretty awesome, though I'm not sure just how difficult it is to use. The video tutorial certainly makes it look very easy (though Bob Ross makes painting look easy, so who knows?). Recolored is free for non-commercial use, Windows only.